While spherical GP lenses can correct astigmatism via the tear lens, often patients will have some amount of residual astigmatism. Front-surface toricity can be added to correct the residual cylinder, but the lens has to be rotationally stable for it to work. Corneal designs must use prism ballast to prevent rotation of the optics. Scleral lenses are typically more stable, especially when toric scleral zones are used to optimize scleral alignment.
An Unexpected Result
Determining the needed front-toric lens power is generally easy with an over-refraction of the best-fitting spherical lens, whether corneal or scleral. But sometimes things aren’t as straightforward as they seem. A recent case illustrates some of the pitfalls that can occur.
I fit my patient with a trial scleral lens (–4.00D) and over-refracted –2.00 –1.25 x 030 to 20/20. I also noted that the lens markings rotated 10° to the left. I then ordered the scleral lens with a prescription of –6.00 –1.25 x 040 (using LARS [left add, right subtract] to determine the axis…left add 10°). At the dispense, the initial acuity was surprisingly only 20/30. The lens fit looked good, but the lens rotation was stabilizing at 5° left instead of the expected 10°.
I was a bit puzzled by the reduced acuity. The lens rotated differently than expected, but that resulted in only 5° of axis misalignment. That should not have impacted acuity very much, as the error would have induced only about –0.25D of residual cylinder. This is based on the well-known fact that for every 5° of misalignment, you would expect about one-sixth of the original cylinder in an over-refraction.
Troubleshooting the Rx
To troubleshoot, first I manually rotated the lens on-eye 10° to the left, but acuity was still only 20/30. After letting the lens go back to its resting position, an over-refraction of –0.25 –0.75 x 172 improved acuity to 20/20.
Toric GP scleral lenses with front-surface toricity are optically similar to soft toric lenses, and you can use the same over-refraction calculators. Using an online calculator, I plugged in the lens prescription, the rotation, and the over-refraction. The calculator suggested a new lens power of –6.50 –1.50 x 025 (assuming that the new lens would also rotate 5° to the left).
I reordered the lens with this power; the new lens fortunately still rotated 5° to the left, and the acuity was 20/20 with a near-plano over-refraction this time.
Prevent Avoidable Errors
For this case, I interpret the error in the first lens to have resulted from a combination of both incorrect rotation measurement and incorrect trial lens over-refraction on the first day. Other possible issues could have been poor trial lens wetting or lens flexure. In some cases, leftover aberrations can make it difficult to get a reliable over-refraction, typically in cases of irregular and/or scarred corneas.
In summary, take some extra time at the initial lens fitting to ensure that your lens rotation assessment and over-refraction are as good as they can be. GP lens optics can be a bit mysterious at times, but with a little diagnostic skill and a good over-refraction calculator, great vision is within reach for your patients. CLS